As for electrophotography apparatus in which an electrostatic latent image is formed on a photoconductive medium (photoconductive drum); a toner is deposited on the charged image; and then transferred and thermally fixed onto a printing medium, such as paper, the dry type using a powder toner has been widely employed.
The powder toner, however, tends to be scattered and often involves the problem of poor resolution due to its particles sizes as large as 7.about.10 .mu.m.
In applications requiring high resolution, therefore, the wet-type using a liquid toner is usually adopted. The liquid toner is less subject to distortion in toner images and can achieve high resolution because it contains toner particles as small as 1 .mu.m and has a large charging capacity.
In the conventional wet-type electrophotography apparatus, a low-viscosity liquid toner obtained by mixing 1.about.2% of toner in an organic solvent has been commonly used as the developing solution. This type of developing solution, however, causes environmental concerns because it contains an organic solvent harmful to the human body and requires a large amount of consumption due to low concentration.
With the conventional wet-type electrophotography apparatus, a solvent that is highly volatile at normal temperatures has been used as the carrier solution, and the excess carrier has been caused to evaporate in the air during the heating and fusing of the toner. That is, the problem of residual carrier solution has been solved without using any special carrier removal equipment. Due to the toxicity and flammability of the solvent used, however, the evaporated carrier has had to be recovered with a large carrier recovery equipment in which it is liquefied for recovery, and the air containing the unrecovered solvent has had to be discharged outdoors with a dedicated ventilation duct. All this has resulted in high cost due to limitations in the site of installation and the large size of equipment.
It was against this background that a wet-type electrophotography apparatus using a high-viscosity, high-concentration developing solution obtained by dispersing a high-concentration toner in silicone oil, etc. was disclosed in International Disclosure Number "WO95/08792." The use of a liquid toner eliminates not only harm to the human body but also the need for large consumption of developing solution due to high toner concentration.
The use of a non-volatile, high-viscosity and high-concentration liquid toner involves prewetting treatment where silicone oil or other prewetting solution is applied to the photoconductive medium prior to the application of the liquid toner so as to prevent the high-viscosity toner from being deposited on the unexposed area of an electrostatic latent image formed on the photoconductive medium. The prewetting solution layer applied during prewetting treatment helps prevent the toner from being deposited on the unexposed area on the photoconductive medium, as shown in FIG. 3. The prewetting solution is an insulating solution having the same or lower viscosity than that of the carrier solution of the developer, and should be applied to a film thickness of not less than 4 .mu.m, or more preferably 8 .mu.m.about.20 .mu.m, to prevent the toner layer on the developing roller from coming direct contact with the photoconductive medium.
According to International Disclosure Number "WO95/08792," the oil used as a prewetting solution should have a viscosity of 0.5.about.5.0 mPa.multidot.S and should be a volatile oil in terms of practical use because a delayed volatilization of the oil remaining on the paper may cause some inconveniences. In other words, it can be said that the process is based on the assumption that the prewetting agent is volatilized to the atmosphere by the heat applied during the fusing process.
This type of process basically involving positive volatilization, however, inevitably limits the service environment of equipment, or has to have equipment of a hermetically enclosed construction and a device for recovering volatile matter and liquefying it by cooling, despite the fact that silicone oil itself is a substance that is harmless to human beings and environment.
The prewetting agent, which is needed for the development process, becomes unnecessary after development. The developing roller comes in contact with the photoconductive medium during development and deposits the toner on the image area by applying a voltage on the roller. After development, the toner image on the photoconductive medium is transferred onto the intermediate transfer medium by the force of electric field. At this time, the prewetting solution and the carrier solution are transferred onto the intermediate transfer medium, together with the developing toner, exerting an influence on the fusion of the toner layer during the fusion process. The toner particles suspending in the prewetting solution are deposited on the background area as a fogged area (depositing of unwanted toner). In this way, the prewetting agent of a predetermined thickness is needed at the time of development using a developing roller, but becomes unnecessary after development. The excess prewetting solution after development reaches an area where the photoconductive medium and the intermediate transfer medium come in contact with each other, and remains there to disturb the image. Furthermore, as the excess prewetting solution is transferred to the intermediate transfer medium, it exerts an influence on the fusion of the toner layer during fixing. The excess prewetting agent should therefore be removed.
Similarly, a predetermined amount of carrier solution in the developing solution is needed to ensure that the developing solution can be supplied to the developing roller in a uniform thickness, and that the toner can be deposited from the developing roller on the image area on the photoconductive medium, but it becomes unnecessary after development. The liquid toner in the liquid developing system can be divided into the solids content and the liquid content. The solids content comprises a thermoplastic resin and pigments, while the liquid content, called the carrier, or carrier solution, includes additives that determine the developing performance and shell stability. The carrier has an important role of transporting fine particles (solids content) of a size about 1/10 of the dry-type development with good dispersion performance throughout the process ranging from development through transfer. The carrier that has outlived its usefulness and remains unremoved up to the next fusion and transfer process could induce imperfect transfer or insufficient fixing. The excess carrier must therefore be removed in some way or other prior to fusion and transfer. Furthermore, the toner particles suspending in the prewetting solution must be removed since they could cause fogging.
Imparting recyclability to this type of non-volatile, high-viscosity and high-concentration liquid toner would lead to greater convenience. In general, the dry-type electrophotography apparatus using powder toner is not based on the use of recyclable powder toner because powder toner can hardly be diluted. In the dry-type electrophotography apparatus, the toner is supplied to the photoconductive medium by supplying the powder toner to the developing roller which is charged by friction with a blade, and the powder toner is replenished to the portion of the developing roller which is transferred to the photoconductive medium.
With the wet-type electrophotography apparatus using liquid toner, on the other hand, the recycling of liquid toner is commonly practiced because the liquid toner can be diluted.
In the wet-type electrophotography apparatus using a low-viscosity liquid toner obtained by mixing 1.about.2% of toner with organic solvent, the concept of recycling organic solvent can be applied, but the concept of recycling liquid toner itself does not hold. Judging from the fact that the liquid toner is fed to the apparatus by spraying it in a large amount to the photoconductive medium due to low toner concentration, the liquid toner itself cannot be recovered. Consequently, even the International Disclosure Number "WO95/08792" does not adopt the concept of recycling liquid toner, following the prior art.
In the case of a high-viscosity, high-concentration liquid toner obtained by dispersing high-concentration toner in silicone oil, etc. however, the concept of recycling can be introduced, unlike other types of liquid toner, since it has entirely different properties from those of liquid toners used in conventional wet-type electrophotography apparatuses. In this case, it is necessary to implement recycling method by taking advantage of the characteristics of the liquid toner such as high viscosity and high concentration.